1. Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device which uses transistors of an identical channel type in a driving device and produces stable emission signals.
2. Description of the Related Art
General organic light emitting display devices, emitting light by electrically exciting a fluorescent organic compound, are designed to display images by voltage- or current-programming a plurality of organic light emitting cells arranged in a matrix. The organic light-emitting cells have diode characteristics, so they are commonly referred to as organic light emitting diodes (OLEDs).
FIG. 1 is a conceptual diagram of an organic light emitting element. Referring to FIG. 1, the organic light emitting element includes an anode which is formed of indium tin oxide (ITO), an organic layer, and a cathode layer which is formed of metal. The organic layer includes an emission material layer (EML), an electron transporting layer (ETL), and a hole transporting layer (HTL).
An organic light emitting cell having an organic light emitting element may be driven by a passive matrix driving method or an active matrix driving method that uses a thin film transistor (TFT) or a MOSFET. In the passive matrix driving method, a line is selected, and a corresponding pixel between an anode and a cathode that overlap each other is driven. In the active matrix driving method, ITO pixel electrodes to which TFTs are connected are driven according to voltages kept in capacitors connected to gates of the TFTs. An active matrix driving method may be categorized as a voltage programming method or a current programming method according to the type of a signal that is applied to store and keep the voltages in the capacitors.
FIG. 2 is a circuit diagram of a pixel circuit to be driven by a conventional voltage programming method. Referring to FIG. 2, a switching transistor M22 is turned on according to a selection signal of a selection scan line Sn, a data voltage from a data line Dm is applied to a gate of a driving transistor M21 via the switching transistor M22, and a difference in potential between the data voltage and a voltage source VDD is stored in a capacitor C21 connected between a source and the gate of the driving transistor M21. Due to the difference in potential, a driving current IOLED flows into an OLED, and thus the OLED emits light. Various gray levels can be displayed by varying the level of the applied data voltage. However, a pixel circuit driven by a conventional voltage programming method may not be able to provide a high gray level because corresponding driving transistors M21 of different pixels may have different threshold voltages and different electron mobilities due to a non-uniform manufacture of the driving transistors.
To overcome this problem, pixel circuits based on a current programming method are used. Substantially identical current sources for supplying currents to the pixel circuits are arranged on the entire surface of a panel (that is, in all data lines of the panel), so that an image of uniform brightness is displayed even when driving transistors of different pixels have different voltage-current characteristics.
FIG. 3 is a circuit diagram of a pixel circuit to be driven by a conventional current programming method. Referring to FIG. 3, when transistors M32 and M33 are turned on according to a selection signal of a selection scan line Sn, a p-channel transistor M31 enters into a diode connection state, so that a current flows across a capacitor C31 and a voltage is stored therein. A gate potential of the transistor M31 is lowered, so that a current flows from a source of the transistor M31 to a drain thereof. When the voltage stored in capacitor C31 increases over time and thus the drain current of the transistor M31 becomes equal to that of the transistor M32, the flow of current across the capacitor C31 is stopped, so that the stored voltage is stabilized. Then, the selection signal of the selection scan line Sn becomes logic high, and thus the transistors M32 and M33 are turned off. However, an emission signal of an emission scan line En becomes logic low, so that a transistor M34 is turned on. Then, power from a voltage source VDD is supplied to the pixel circuit, and a driving current IOLED corresponding to the voltage stored in the capacitor C31 flows into an OLED, so that the OLED emits light with a certain brightness.
In order to drive a pixel circuit according to the conventional current programming method as illustrated in FIG. 3, a driving device for supplying a signal to a selection scan line and a driving device for supplying a signal to an emission scan line are required. In a conventional organic light emitting display device, different channel types of CMOS transistors are used in the driving devices for supplying signals to the selection scan line and the emission scan line. The use of the different channel types of CMOS transistors complicates the manufacture of the display device and increases manufacturing costs.